1. Field of the Invention
The present invention relates to an optical transmitter receiver for performing optical communication over, for example, an optical transmission medium. More particularly, this invention is concerned with an optical transmitter receiver having a power-of-transmitted light control means for setting the power of light to be transmitted from the optical transmitter receiver to a proper value.
2. Description of the Related Art
An optical transmitter receiver in accordance with the present invention is configured as, for example, shown in FIG. 1.
Referring to FIG. 1, paired optical transmitter receivers 1a and 1b are optically connected to each other over an optical fiber 3, which is an optical transmission medium, between two apparatuses 2a and 2b. The optical transmitter receivers la and 1b are connected to the apparatuses 2a and 2b with physical layer control apparatuses 4a and 4b between them.
The optical transmitter receivers 1a and 1b have the same configuration. The internal configuration of the optical transmitter receiver 1b shown on the right-hand side of FIG. 1 is therefore not illustrated.
The optical transmitter receiver 1a shown on the left-hand side of FIG. 1 will be described below.
Referring to FIG. 1, the optical transmitter receiver 1a consists of a transmitter 5 and a receiver 6.
The transmitter 5 is, as illustrated, composed of a laser diode 5a that is a light emitting device, a drive circuit 5b for driving the laser diode 5a, and a shutdown circuit 5c for controlling driving performed by the drive circuit 5b. 
The laser diode 5a converts an input signal into a laser beam whose intensity is proportional to the level of the input signal, and transmits the laser beam to the other optical transmitter receiver 1b over the optical fiber 3.
Moreover, the drive circuit 5b drives the laser diode 5a to luminescence according to transmission data Tx Data fed from the physical laser control unit 4a. 
The shutdown circuit 5c actuates the drive circuit 5b according to a transmission instruction signal Tx Enable fed from the physical layer control apparatus 4a. 
In contrast, the receiver 6 is, as shown in FIG. 1, composed of a photodiode 6a serving as a light receiving device, a current-voltage amplifier 6b, a post-amplifier 6c, a peak hold circuit 6d, and a signal detection circuit 6e. 
The photodiode 6a receives a light signal sent-from the other optical transmitter receiver 1b (shown on the right-hand side in the drawing) over the optical fiber 3, and outputs an electric signal proportional to the light signal.
The current-voltage amplifier 6b amplifies an output signal of the photodiode 6a. 
The post-amplifier 6c amplifies an output signal of the current-voltage amplifier 6b, and transmits a resultant signal as reception data Rx Date to the physical layer control apparatus 4a. 
The peak hold circuit 6d detects the peak of the output signal of the current-voltage amplifier 6b. 
The signal detection circuit 6d detects a signal output from the peak hold circuit 6d and transmits a reception acknowledgement signal to the physical layer control apparatus 4a. 
The physical layer control apparatus 4a controls communications according to a protocol whose level is higher by one step than the protocol according to which the optical transmitter receiver 1a performs communications. Transmission data Tx Date and a transmission instruction signal Tx Enable are sent from the apparatus 2a to the optical transmitter receiver 1a. The optical transmitter receiver 1a transmits reception data Rx Data and a reception acknowledgement signal Rx SD to the apparatus 2a. 
When the optical transmitter receivers 1a and 1b have the foregoing components, if the apparatus 2a outputs transmission data to the associated physical layer control apparatus 4a, the physical layer control apparatus 4a feeds the transmission instruction signal Rx Enable to the shutdown circuit 5c in the transmitter 5 of the optical transmitter receiver 1a. Consequently, the drive circuit 5b is actuated.
When the physical layer control apparatus 4a feeds transmission data Tx Date to the drive circuit 5b in the transmitter 5 of the optical transmitter receiver 1a, the drive circuit 5b drives the laser diode 5a according to the transmission data. This causes the laser diode 5a to emit light whose intensity is proportional to the transmission data.
A laser beam emitted from the laser diode 5a included in the other optical transmitter receiver 1b falls on the photodiode 6a in the receiver 6 of the optical transmitter receiver 1a over the optical fiber 3. This causes the photodiode 6a to output an electric signal proportional to the incident light. The electric signal is amplified by the current-voltage amplifier 6b, and then further amplified by the post-amplifier 6c. An output signal of the post-amplifier 6c is input as reception data Rx Data to the physical layer control apparatus 4a. An output signal of the current-voltage amplifier 6b has its peak detected by the peak hold circuit 6d. A reception acknowledgement signal Rx SD is input to the physical layer control apparatus 4a via the signal detection circuit 6e. The physical layer control apparatus 4a transmits reception data to the apparatus 2a. 
The internal configuration of the optical transmitter receiver 1b is identical to that of the optical transmitter receiver 1a. The apparatus 2a transmits transmission data Tx Data to the other apparatus 2b, and the other apparatus 2b transmits transmission data Tx Data to the apparatus 2b. Consequently, optical communication is performed between the apparatuses 2a and 2b. 
A description will be made of an optical fiber adopted as the optical fiber 3 and characterized by an optical transmission loss of 0.1 dB/m relative to any wavelength xcex of light to be transmitted, for example, 650, 780, 850, 1300, 1500, or 1550 nm.
Specifically, a plastic optical fiber is adopted. The sensitivity of the photodiode 6a is set to 0.5 A/W, and a gain to be produced by the current-voltage amplifier 6b is set to 4 kxcexa9. A maximum amplitude of a signal that can be output from the current-voltage amplifier 6a is set to 1 Vp-p, and a minimum amplitude of a signal that can be input to the post-amplifier 6c is set to 0.2 Vp-p. In practice, for example, a silicon pin photodiode and a bipolar chip set will do.
Since the maximum output level of the current-voltage amplifier 6b is 1 Vp-p and the minimum input level of the post-amplifier 6c is 0.2Vp-p, a dynamic range offered for an output of the current-voltage amplifier 6b is from 0.2 to 1.0 Vp-p.
Since a gain to be produced by the current-voltage amplifier 6b is 4 kxcexa9, a dynamic range offered for an input of the current-voltage amplifier 6b is from 0.05 to 0.25 mAp-p.
Since the sensitivity of the photodiode 6a is 0.5 A/W, a dynamic range offered for an input of the photodiode 6a is from 0.1 to 0.5 mWp-p.
Assuming that light received by the optical transmitter receiver 1a or 1b falls on the photodiode with the power thereof 100% maintained, a dynamic range offered for received light by the optical transmitter receiver 1a or 1b is from 0.1 to 0.5 mWp-p.
Assume that the optical fiber 3 has lengths ranging from 0 to 70 m. In this case, when the power of light transmitted from the optical transmitter receiver 1a or 1b is 0.5 mWp-p, the power of light received by the other optical transmitter receiver 1b or 1a varies as expressed with a curve E in FIG. 2. When the optical fiber 3 has a length of 0 m, the power of received light is 0.5 mWp-p. When the optical fiber 3 has a length of 70 m, the power of received light is 0.1 mWp-p. This range of powers agrees with the aforesaid dynamic range offered by the optical transmitter receiver.
Assuming that the optical fiber 3 has lengths ranging from 30 to 100 m, the power of received light varies as expressed with a curve F in FIG. 2, though the power of light to be transmitted remains 1.0 mWp-p. Specifically, when the optical fiber 3 has a length of 30 m, the power of received light is 0.5 mWp-p. When the optical fiber 3 has a length of 100 m, the power of received light is 0.1 mWp-p. This range of powers agrees with the aforesaid dynamic range offered by the optical transmitter receiver.
Assuming that the power of light to be transmitted remains constant, unless a distance of optical transmission permitted by the optical fiber 3 falls below 70 m, the power of received light falls outside the dynamic range offered by the optical transmitter receiver. When the distance of optical transmission exceeds 70 m, the power of light to be transmitted must be changed from one value to another.
The optical transmitter receiver capable of changing the power of light to be transmitted is configured as shown in FIG. 3.
Referring to FIG. 3, the optical transmitter receiver 1a or 1b is different from the optical transmitter receiver 1a or 1b shown in FIG. 2 in a point that the optical transmitter receiver has a controller 7.
The controller 7 consists of a power-of-received light detection circuit 7a and a modulation control circuit 7b. The power-of-received light detection circuit 7a inputs a detection signal from the peak hold circuit 6d in the receiver 6. The modulation control circuit 7b inputs a power-of-received light detection signal from the power-of-received light detection circuit 7a. 
The power-of-received light detection circuit 7a makes a judgment as described later according to a peak value detected by the peak hold circuit 6d, and outputs a power-of-received light detection signal RxPower Detect to the modulation control circuit 7b. 
The modulation control circuit 7b controls modulation performed by the drive circuit 5b according to the power-of-received light detection signal output from the power-of-received light detection circuit 7a, and sets the power of light to be transmitted to either of two values.
The optical transmitter receiver 1a or 1b having the foregoing components performs optical communication in the same manner as the optical transmitter receiver 1a or 1b shown in FIG. 1 does.
Assume that the optical transmission loss is 0.1 dB/m and the dynamic range for received light is from 0.1 to 0.5 mWp-p. In this case, when the optical fiber 3 has lengths ranging from 0 to 60 m, the power of received light varies as expressed with a curve E shown in FIG. 4, though the power of light to be transmitted remains 0.500 mWp-p. Specifically, when the optical fiber 3 has a length of 0 m, the power of received light is 0.500 mWp-p. When the optical fiber 3 has a length of 60 m, the power of received light is 0.126 mWp-p.
When the power of received light is 0.126 mWp-p or less, the peak value of an output signal of the current-voltage amplifier 6b is 0.251 Vp-p or less. At this time, the power-of-received light detection circuit 7a judges that the power of received light is too low, and transmits a high-level signal as a power-of-received light detection signal to the modulation control circuit 7b. The power of light to be emitted from the laser diode 5a is thus changed to 1.0 mWp-p (See an arrow E1 in FIG. 4).
When the optical fiber 3 has lengths ranging from 60 to 100 m, the power of received light varies as expressed with a curve F in FIG. 4, though the power of light to be transmitted remains 1.000 mWp-p. Specifically, when the optical fiber 3 has a length of 60 m, the power of received light is 0.251 mWp-p. When the optical fiber 3 has a length of 100 m, the power of received light is 0.100 mWp-p. The power of received light falls within the aforesaid dynamic range offered by the optical transmitter receiver.
When the optical fiber 3 has lengths ranging from 100 to 40 m, the power of received light varies as expressed with a curve F in FIG. 4, though the power of light to be transmitted remains 1.000 mWp-p. Specifically, when the optical fiber 3 has a length of 100 m, the power of received light is 0.100 mWp-p. When the optical fiber has a length of 40 m, the power of received light is 0.398 mWp-p.
When the power of received light is 0.398 mWp-p or more, the peak value of an output signal of the current-voltage amplifier 6b is 0.398 Vp-p or more. At this time, the power-of-received light detection circuit 7a judges that the power of received light is too high, and transmits a low-level signal as a power-of-received light detection signal to the modulation control circuit 7b. The power of light to be emitted from the laser diode 5a is thus changed to 0.500 mWp-p (see an arrow F1 in FIG. 4).
When optical fiber 3 has lengths ranging from 40 to 0 m, the power of received light varies as expressed with a curve E in FIG. 4, though the power of transmitted light remains 0.500 mWp-p. Namely, when the optical fiber 3 has a length of 40 m, the power of received light is 0.199 mWp-p. When the optical fiber 3 has a length of 0 m, the power of received light is 0.500 mWp-p. The power of received light falls within the dynamic range offered by the optical transmitter receiver.
The power of light to be transmitted is changed between two values according to the peak value of the power of received light. Thus, as long as a distance of optical transmission is 100 m or less, the power of received light falls within the aforesaid dynamic range offered by the optical transmitter receiver.
However, the optical transmitter receiver 1a or 1b capable of changing the power of light to be transmitted shown in FIG. 3 has drawbacks described below.
Assume that the optical fiber 3 has a length of 80 m and the optical transmitter receiver 1a starts optical communication. In this case, as seen in FIG. 5 and FIG. 6, when the power of light transmitted initially is 0.500 mWp-p, the power of light received by the optical transmitter receiver 1b is 0.079 mWp-p. The power of received light, that is, 0.079 mWp-p is lower than 0.126 mWp-p that is a criterion specified in the power-of-received light detection circuit 7a. 
The power-of-received light detection circuit 7a therefore judges that the power of received light is too low, and outputs a high-level signal as a power-of-received light detection signal to the modulation control circuit 7b. Consequently, the power of light to be transmitted from the optical transmitter receiver 1b is changed from 0.500 mWp-p to 1.000 mWp-p.
Thereafter, when the optical fiber 3 has a length of 80 m, the power of light transmitted from the optical transmitter receiver 1b is 1.000 mWp-p, and the power of light received by the optical transmitter receiver 1a is 0.158 mWp-p. The power of received light, that is, 0.158 mWp-p is larger than 0.126 mWp-p. The power-of-received light detection circuit 7a in the controller 7 of the optical transmitter receiver 1a therefore misjudges that the distance of optical transmission is 50 m (the optical fiber 3 has a length of 50 m), and that the power of received light is optimal. Consequently, the power-of-received light detection circuit 7a outputs a low-level signal as a power-of-received light detection signal to the modulation control circuit 7b. 
Since the power of light transmitted from the optical transmitter receiver 1a remains 0.500 mWp-p, the power of light received by the optical transmitter receiver 1b is 0.079 mWp-p. The power of received light is lower than 0.100 mWp-p, and falls outside the dynamic range offered by the optical transmitter receiver.
As mentioned above, when the optical transmitter receiver 1a or 1b changes the power of light to be transmitted on its own judgment, the power of light to be transmitted becomes different between the paired optical transmitter receivers. This disables appropriate optical communication.
Accordingly, an object of the present invention is to provide an optical transmitter receiver in which the power of light to be transmitted therefrom is set to a proper value by controlling the relative timing of changing the power of light to be transmitted therefrom from one value to another.
According to the present invention, the foregoing object is accomplished with an optical transmitter receiver consisting of an optical transmitter, an optical receiver, a signal level detecting means, and a signal level control means. The optical transmitter transmits a light signal to a mate optical transmitter receiver. The optical receiver receives a light signal from the mate optical transmitter receiver. The signal level detecting means detects the level of the light signal received by the optical receiver. The signal level control means changes the power of the light signal to be transmitted from the optical transmitter from one value to another according to the signal level detected by the signal level detecting means. The optical transmitter receiver further includes a delay means for delaying changing of the power of a light signal to be transmitted which is performed by the signal level control means.
Owing to the above configuration, the optical transmitter receiver has the delay means for delaying changing of the power of a light signal to be transmitted which is performed by the signal level control means. Changing of the power of a light signal to be transmitted is therefore delayed.
The optical transmitter receiver can therefore transmit a light signal to the mate optical transmitter receiver without changing the power of the light signal to be transmitted. Thereafter, the optical transmitter receiver uses the signal level detecting means to detect the power of light received from the mate optical transmitter receiver, and changes the power of light to be transmitted to an optimal value.
Unlike the conventional optical transmitter receivers, it will not take place that one of optical transmitter receivers changes the power of light to be transmitted on its own judgment.
The above object is accomplished with a method of optical transmission and reception comprising a transmission step, a reception step, a signal level detection step, a delay step, and a signal level control step. At the transmission step, a light signal is transmitted to a mate optical transmitter receiver. At the reception step, a light signal is received from the mate optical transmitter receiver. At the signal level detection step, the level of the received light signal is detected. At the delay step, the signal detected at the signal level detection step is delayed. At the signal level control step, the power of a light signal to be transmitted at the transmission step is changed based on the level of the signal delayed at the delay step.
According to the method, the signal detected at the signal level detection step is delayed at the delay step. Changing of the power of a light signal to be transmitted is delayed.
A light signal is therefore transmitted to a mate optical transmitter receiver without the necessity of changing the power of the light signal to be transmitted. Thereafter, the power of a light signal to be transmitted at the transmission step is changed based on the level of the signal delayed at the delay step.
It will therefore not take place that the mate optical transmitter receiver changes the power of light to be transmitted on its own judgment.